The Indian Ocean Sea Floor

Plates grow by pulling apart at the mid-ocean ridges and sinking back into
the earth's interior at the tranches, mostly around the rim of the Pacific. As
plates grow, they create strips of progressively older crust on either side of
the ridge. We can determine the age of the ocean floor by drilling into it an
retrieving samples, but mostly because newly formed crust is slightly magnetized
by the earth's magnetic field, and we can compare the magnetism of the sea floor
with the history of changes in the earth's magnetic field.

The earth has two kinds of crust. The continents are mostly made of thick
granite. When continents pull apart, the gap is filled by thin crust made of
basalt. In plate tectonics, a continent is any piece of continental crust
surrounded by oceanic crust or plate boundaries. New Zealand is a continent.
Unlike most continents, most of New Zealand is submerged. Also, unlike Africa
and South America, it does not fit neatly against Australia. The submerged
northern portion, called the Lord Howe Rise, is actually a collection of smaller
fragments that moved in a complex way so the present shape is not the original
shape.

The ocean floors show a remarkable degree of regularity. The crest of the
mid-ocean ridges is at an average depth of 2700 meters, and the 3000 meter
contour on the map reveals most of the crest. As newly formed ocean crust cools,
it becomes denser and sinks, until it finally sinks to an average depth of 5500
meters. The broad deep areas about 5000-6000 meters below sea level are called
abyssal plains and are the flattest areas on Earth. The very deepest parts of
the ocean, up to 11 kilometers deep, make up a very tiny part of the sea floor.
They are not out in the middle, like one might expect in a bowl. Instead they
are long, narrow trenches close to a continent or island chain. The trenches
mark convergent plate boundaries where oceanic crust sinks back into the earth's
interior.

Antarctica was part of eastern Gondwanaland along with Madagascar, India and
Australia. Eastern Gondwanaland broke off of Africa about 180 million years ago,
and not long afterward, India and Madagascar broke off. Finally, Australia and
New Zealand broke off around 70 million years ago. As a result, Antarctica is
almost entirely surrounded by divergent plate boundaries. The remarkably
straight ridge between India and Australia is called the Ninety East Ridge
because it runs along 90 degrees east longitude. It is a former plate boundary
between India and Australia. The separate collisions of India and Australia with
Eurasia are causing the Indian-Australian Plate to break up, but the new break
cuts across the Ninety East Ridge nearly at right angles.

Possible Coloring

Labeled Features

In addition to the large plates, significant small plates are labeled. Very small plates
are not labeled.